Arc welding is a technique in which electric power is supplied via a torch to a welding wire that is continuously fed to generate arc discharge between the welding wire and an object to be welded, so that the welding wire and a part of the object to be welded are melted and bonded together by the heat energy of the arc discharge. Art described in Patent Document 1 is known as related art associated with a torch used for such arc welding.
Patent Document 1 describes an arc-welding torch including: a wire feeding portion that bends a welding wire to a predetermined curvature to feed it; and a power supply torch portion that is provided continuous to the wire feeding portion and supplies electric power via a contact tip to the welding wire fed from the wire feeding portion. The arc-welding torch is characterized in that the power supply torch portion includes a guide cylinder that guides the welding wire substantially linearly to feed it to the contact tip.
Further, among such arc-welding torches, an arc-welding torch is known that includes: a contact tip 10′ having, on a distal-end inner circumferential surface thereof, a power supply portion 10a′ that contacts a fed welding wire 2 to supply current; and a wire guide 11′ that is provided having a predetermined clearance within the contact tip 10′ and guides the fed welding wire 2, as shown in FIG. 8. The arc-welding torch is configured such that axes of the contact tip 10′ and the wire guide 11′ are relatively changed so as to cause the welding wire 2 guided by the wire guide 11′ to contact the power supply portion 10a′ of the contact tip 10′. The arc-welding torch is also configured such that inert gas is supplied to a flow path formed between an outer side of the contact tip 10′ and a nozzle 13′ and is blown to the welded portion. In such an arc-welding torch 1′, the wire guide 11′ guides the welding wire 2. Therefore, it is possible to accurately feed the welding wire 2 to a welding site. Moreover, the axes of the contact tip 10′ and the wire guide 11′ are relatively changed so as to cause the welding wire 2 guided by the wire guide 11′ to press-contact the power supply portion 10a′ of the contact tip 10′. Therefore, current can be securely supplied to the welding wire 2. The contact tip 10′ of such an arc-welding torch 1′ is generally formed such that, when the welding wire 2 has a diameter of 1.2 mm for example, the power supply portion 10a′ has an axial length La′ of 8.7 to 11.1 mm as shown in FIG. 9A, in order to securely contact the power supply portion 10a′ to the welding wire 2.
In the arc-welding torch 1′, when the contact tip 10′ is new, a power supply point P is generally set at a distal end of the power supply portion 10a′ of the contact tip 10′ on a base material 3 side, in order to supply current to the welding wire 2 as efficiently as possible (see FIG. 9A). However, since the welding wire 2 is fed while contacting the power supply portion 10a′ of the contact tip 10′, the power supply portion 10a′ wears as shown in FIG. 9B. As a result, a position at which the welding wire 2 contacts the power supply portion 10a′ (this position is referred to as the power supply point P) shifts backward from the distal end of the contact tip 10′ (in a direction opposite to the feeding direction of the welding wire).
Here, a change in the welding current in accordance with a change in the distance between the power supply point P and the base material 3 from Ka′ to Kb′ will be described, based on FIG. 10. The graph in FIG. 10 shows the relationship between the welding current and the distance between the distal end of the contact tip 10′ and the base material 3 that has changed from L1 to L2 (i.e., the distance between the power supply point P and the base material 3 that has changed from Ka′ to Kb′), assuming that the power supply point P of the welding wire 2 having a diameter of 1.2 mm is always located at the distal end of the power supply portion 10a′ of the contact tip 10′ on the base material 3 side. It is clear from the graph that, with an increase in the distance between the distal end of the contact tip 10′ and the base material 3, the length of the welding wire 2 through which the current from the power supply point P flows also increases. Thus, electrical resistance increases accordingly, thereby decreasing the welding current. In the case shown in FIG. 10, when the distance between the distal end of the contact tip 10′ and the base material 3 increases by 3 mm from L1 (=15 mm) to L2 (=18 mm), the welding current falls by 20 A from approximately 177 A to 157 A. If the welding current falls at least 20 A due to wear of the contact tip 10′ as described above, welding defects such as incomplete penetration, insufficient leg length or throat thickness, and the like may occur.
The contact tip 10′ wears after use for a predetermined length of welding, and thus, the initial welding current falls to no more than a predetermined lower limit current (a minimum current required for maintaining welding quality, that differs depending on welding conditions). In the case shown in FIG. 11, at the welding length of 21 m indicated by an arrow where the welding current falls from the initial current of 187 A to 157 A, values fall below the predetermined lower limit current in this case.
Therefore, in the related art, the production in the welding process is temporarily stopped before the welding current falls below the predetermined lower limit current, i.e., at 16 m, which is 5 m before the welding length reaches 21 m (see the vertical line affixed with an arrow in FIG. 11), and the contact tip 10′ is periodically replaced at short intervals. If the periodical replacement cannot be appropriately conducted, welding sites are inspected in order to prevent welding defects, and any welding defect found in the inspection is repaired.    Patent Document 1: Japanese Patent Application Publication No. JP-A-05-269580